Fill your images with a lighting effect. This is very handy when dealing with darker images. How? Set the effect parameter to fill_light (e_fill_light in URLs). You can control the fill light level, the range is between -100 and 100. The default value is 0.

Here's an original image. Notice that the image is a little dark

Set the effect parameter to fill_light (e_fill_light in URLs) to add some lighting to the image:

While the default level of the 'Fill light' effect is 0, you can customize the level. For example, by setting the effect parameter to fill_light:80 (e_fill_light:80 in URLs) :